The present invention relates generally to touchpad devices, and more particularly relates to touchpad devices which detect both touch and pressure.
Touch sensing devices are well known, and take a number of different forms. Perhaps the best known are resistive-membrane position sensors, which have been used in a number of different applications for many years and basically respond to pressure. Such devices have been used as keyboards, position indicators, and so forth. Other types of touch sensing devices include resistive tablets, surface acoustic wave devices, touch sensors based on strain gages or pressure sensors, and optical sensors. The main advantage of pressure sensitive touch pads is that they can be used with a pen, and many applications for pressure sensitive involve their use as pen "digitizers," often as data entry devices.
Pressure sensitive position sensors have historically offered little benefit for use as a pointing device (as opposed to a data entry or writing device) because the pressure needed to make them operate inherently creates stiction between the finger and the sensor surface. Such stiction has, in large measure, prevented such devices from becoming popular.
Yet another touch sensing technology is capacitive sensing, in which the location of a finger (or in some instances another object such as a stylus) over a sensing device is determined by virtue of variations in capacitance under and around the location of the finger. Typical of capacitive touch sensing devices are touch screens and capacitive pads which employ a matrix of row and column electrodes and detect, for example, either the transcapacitance between row and column electrodes or the effective capacitance to virtual ground. Other capacitive techniques are also known. Some touch sensitive devices are known to use interpolation for more precisely identifying the location of a finger or a specialized stylus.
Capacitive touch sensing devices require little or no pressure, thus eliminating the stiction problem. This has led to them becoming accepted as very good pointing devices. Nevertheless, the capacitive sensing techniques used in such sensors typically require a relatively wide contact--i.e., a finger as opposed to the tip of a pen--which makes them less desirable for the classical data entry/digitizer application that is well suited to a stylus.
U.S. Pat. No. 5,543,590 discloses an example of a touchpad which capacitively detects a finger, and can also detect the pressure applied by the finger by detecting the size of the area of the touchpad which capacitively detects the finger. Assuming equal sized fingers on users, a larger area of capacitive detection is assumed to mean more pressure is applied.
While some recently developed touch pads are highly sophisticated and can detect very light touch, some users occasionally wish to use pens or other styli to contact the touch pad. Others primarily wish to use a stylus, but occasionally want to use touch. As a result, there is a need for a touch pad capable of high sensitivity both to detecting capacitively a finger touch and also detecting the small pressure footprint of a stylus.